Pico CTF

Binary Search

Description

Want to play a game? As you use more of the shell, you might be interested in how they work! Binary search is a classic algorithm used to quickly find an item in a sorted list. Can you find the flag? You'll have 1000 possibilities and only 10 guesses. Cyber security often has a huge amount of data to look through - from logs, vulnerability reports, and forensics. Practicing the fundamentals manually might help you in the future when you have to write your own tools! You can download the challenge files here:

ssh -p 52311 ctf-player@atlas.picoctf.net Using the password 1db87a14. Accept the fingerprint with yes, and ls once connected to begin. Remember, in a shell, passwords are hidden!

Hints

  1. Have you ever played hot or cold? Binary search is a bit like that.
  2. You have a very limited number of guesses. Try larger jumps between numbers!
  3. The program will randomly choose a new number each time you connect. You can always try again, but you should start your binary search over from the beginning - try around 500. Can you think of why?

Solution

picoCTF{g00d_gu355_1597707f}

CLI

kappa@kappa-Aspire-Z5600:~$ ssh -p 52311 ctf-player@atlas.picoctf.net
ctf-player@atlas.picoctf.net's password:
Welcome to the Binary Search Game!
I'm thinking of a number between 1 and 1000.
Enter your guess: 500
Higher! Try again.
Enter your guess: 750
Higher! Try again.
Enter your guess: 825
Higher! Try again.
Enter your guess: 900
Lower! Try again.
Enter your guess: 840
Lower! Try again.
Enter your guess: 830
Higher! Try again.
Enter your guess: 835
Congratulations! You guessed the correct number: 835

Here's your flag: picoCTF{g00d_gu355_1597707f}

Connection to atlas.picoctf.net closed.
kappa@kappa-Aspire-Z5600:~$